Transconductor employing line type field controlled semiconductor



Nov. 18, 1952 o. M. sTUi-:TZER 2,618,690

TRANscoNoUcToR EMPLOYING LINE TYPE FIELD goNTRoLLED sEMIcoNnUc-TQR Filed oct. e, 1949 BY @web /ff la Mm% Pmme Nov'. 1s, 1952 TRANBCONDUCTOR EMPLOYIG LINE TYPE FIELD CONTROLLED SEMICONDUCTOR Otmar M. Stuetzer, Dayton, Ohio Application October 6, 1949, Serial No. 119,985

1 Claim.

(Cl. F75-366) (Granted under rthe act of March 3, 1883, u amended April 39, 1928; 370 0. G. 757) The invention described herein may be manulectured and used by or for the Government for governmental purposes without payment to be of any royalty thereon.

wlhis invention relates to transconductlve devices of the type employing a semiconductor as the current controlling element.

yin my patent application Serial No. 119,541, died October 4, 1949 there was described and claimed a transconductive device employing a ceconductor in which the output electrode made a point contact with the semiconductor and in which the input or control electrode was positioned very close to the point contact and served to control the electrical fleld in the neighborhood oi the point contact. Variations in control electrode potential, as by a signal voltage, produced corresponding variations in output electrode current. e device had high input impedance, high current and power amplication, and high efficiency. lt had circuit impedance values comparaiole to those of a vacuum tube and within its power limitations could, like a vacuum tube, be

used as on amplifier. detector, oscillator, volume controlling device, grid controlled rectifier, etc.

e device also exhibited a peculiar characterictlc in that the sign oi its mutual conductance was a iunction ci thecontrol electrode bias voltare. This characteristic permitted its use as a phase inverter in which a reversal of phase could he produced hy shifting the bias from a value at which the mutual conductance was of one sign to a value at which the mutual conductance was of the opposite sien.

The transconductive device described in the present application is similar to that described above in. its characteristics and uses but is dii'- ierent in that a line contact, rather than a point contact, is made between the output electrode and the semiconductor, a line contact being defined as a contact area one dimension of which is very large compared to the other. This type construction is easier, particularly for quantity production oi' the device, and in addition results in smaller intcrelectrode capacities, higher mechanical and electrical stability and higher power handling capacity.

It is accordingly the object of the invention to provide a transconductive device employing .a field controlled semiconductor and having many oi the characteristics of a vacuum while not requiring a heated cathode or an evacuated envelope.

It is a further object of the invention to provide. a transconductlve device of the semiconductor. type having a high input impedance, high current and power ampliiication and high eiliciency.

It is a still further object of the invention to provide a transconductive device of the semiconductor type in which a line contact exists between the output electrode and the semiconductor and in which a control electrode is provided for controlling the electrical field in the neighborhood oi' the line contact.

It is another object oi the invention to provide a transconductive device of the semiconductor type the design oi which lends itself readily to modern manufacturing techniques such as evan eration, sputtering and electroplating.

It is still another object of the invention to provide a transconductive device oi the seconductor type which has a high decree oi mecha ical and electrical stability.

It is a further object oi the invention to devise suitable processes for manufacturing transconductive devices of the type described.

The specic detalls of the invention will he given in connection with the accompanying drawings, showing preferred embodiments oi the invention, in which v Fig. l is a schematic diagram of a transconduc tive device employing line contact between the output electrode and the semiconductor;

l'ig. 2 is a sectional view of Fig. l

Fig. 3 shows a practical embodiment oi the an' paratus shown in Figs.- l and il;

Fig. 4 shows an alternative electrode structure; and

Fig. 5 is a sectional vieu.r oi Fig. d showing also a semiconductor in cooperative relation with the electrode structure oi Fig. d.

Referring to Fig. l a transconductive device in 4 accordance with the invention is shown d comprises a semiconductor i9, a control electrode il and an output electrode i2. The output electrode i2 consists of a very fine wire which is pressed against the semiconductor l0 by the control electrode il The control electrode is insulated irom the wire l2 by a sheet of insulating material it which should be as thin as possible. .A drop oi colloidal silver i4 is deposited on the insulating sheet and around the end of the control electrode to distribute the electrical potential of the control electrode over the insulating member. As semiconductor material n-type and p-type germanium, p-type silicon and tellurium are recommended. The n-type and p-type designation is in accordance with the present theory of conduction in semiconductors, the former representing conduction by free electrons and the latter representing conduction by holes due to absences of electrons in the interatomic bonds. The output electrode I2 is shown in Fig. 1 as the core of a piece of Wollaston wire. Wollaston wire is made up of a very ilne wire of relatively hard metal such as platinum as the core and an outer sheath I5 oi a relatively soit metal such as silver which protects theilne core wire and adds sumcient strength and size to make the composite strand easy to handle. The core may have a diameter of from 2% to 5 microns and is exposed by eating away the outer layer oi'silver with a suitable acid usually supplied by the manufacturer of the wire. While Wollaston wire is well suited for use as the output electrode, fine wires oi' other metals such as tungsten may also be employed. The insulating sheet I3 may be made oi' any suitable thin insulating material such as thin glass, paper or plastic material such as cellophane. The control electrode should have a ilat end surface of greater dimensions than the diameter oi' the wire forming the output electrode.

An input circuit, consisting of input terminals I 6 and biasvoltage source I'I, is connected between the control electrode and the semiconductor. An output circuit, consisting of load impedance I8 and direct current source I9. is connected between the output electrode and the semiconductor. Variation of the potential of control electrode II, as by the application of a signal to terminals I6, results in corresponding variations of current in the output circuit. This is believed to be due to the variation of the electric field in the semiconductor in the neighborhood of the line contact. The polarity of source I 9 is normally such as to send current in the high resistance or back direction of the semiconductor. For n-type germanium operation in the back direction requires that the output electrode be negative with respect to the semiconductor as shown in Fig. l. Operation in the forward direction is also possible and results in low noise and low relaxation effects making such operation advantageous in some high frequency applications. Forward operation is also characterized by greatly reduced power output and output impedance as compared with operation in the back direction. The input impedance of the device is extremely high and comparable to that of a vacuum tube. The device also draws no current from bias source I1 which adds to its eniciency.

Fig. 3 shows a practical form which a transconductive device in accordance with the inven- .tion may take. The device shown is mounted on a standard octal tube base. A crystal I0 of semicon-ductive material is mounted on a holder 2l) which is in turn mounted on pin 2I of the socket.l

The fine wire I2, which is shown as the core of a piece of Wollaston wire having an outer sheath I5, is pressed against the surface of the crystal by control electrode II. The spring support 22 serves to force the control electrode toward the crystal and also to electrically connect the electrode to pin 2l. A small thin sheet of insulating material I3 is placed between the control electrode and the output electrode and a drop of Acolloidal silver I4 is placed on the insulating sheet at the end of the control electrode for the purpose of extending the effective area of the electrode so as to control the field in the neigh borhood of the line contact between the wire I2 and thesemiconductor III over a considerable length of the wire. 'I'he output electrode ls'electrically connected to pin 24.

A diiierent type output and control electrode assembly is shown in Fig. 4. A series of parallel connected metallic output electrodes 30 and a series of parallel connected metallic control electrodes l I, positioned on either side of the output electrodes, are mounted on a piece oi insulating material l2. Leads Il and 34 serve to make electrical connection to the output and control electrodes respectively. 'I'he input electrodes II are ush with the insulating base 32 but the output electrodes I0 extend slightly above the surface of the base as shown in the sectional view of Fig. 5. When this electrode assembly is pressed against the fiat surface of a semiconductor I0, as shown in Fig. 5, the output electrodes I0 make line contacts with the semiconductor but the controlelectrodes 3I are slightly spaced from the semiconductor so that they do not touch and are able to control the electrical field in the neighborhood of the line contacts between electrodes 30 and the semiconductor. The input electrodes 3| and the output electrodes 3U may be connected in input and output circuits similar to those connected to control electrode II and output electrode I2 in Fig. 1. It is evident that the output electrode structure of Fig. 4 provides in effect a number of line contacts in parallel which makes possible higher output current values and higher power output than in the case of Fig. 1.

In order to effectively control the electrical iield in the semiconductor in the vicinity of the line contact it is necessary that the sizes of the output and control electrodes and their spacings be extremely small. Accordingly. the output electrodes 30 and the control electrodes 3|, and also the spacings between these electrodes, are made to be of the order of 1 to 2 microns. In this respect it is pointed out that, for the sake oi convenience, Figs. 4 and 5 are not drawn to scale, and that in an actual embodiment of the electrode assembly the block of insulation 32. must necessarily be much greater in size in comparison to the electrodes than indicated in these figures. The electrode assembly may be marde by first engraving the grid-like structure on a fiat surface of suitable insulating material such as quartz. The engravings should be 1 to 2 microns wide and equally deep. The spacings between the engraved grooves for the control and output electrodes should also be from l to 2 microns. Engraving of this character can be accomplished by the use of a dividing machine such as employed in making optical grids. After engraving a metal such as silver or antimony is applied over the entire engraved area by evaporation or sputtering. The surface of the insulator is then ground oil to remove all metal` except that remaining in the engraved grooves. 'I'he assembly is thenv placed in anr electrolytic solution and the output electrodes 30, but not the control electrodes 3i connected in an electroplating circuit. 'I'he resulting plated metal on the output electrodes raises them above the surface of the insulator as shown in Fig. 5. A suitable dielectric layer, such as a thin film of oil or glycerine or other material having a high dielectric constant, may be inserted between the electrode assembly and the semiconductor if desired and the presence of such a film has been found to increase the mutual conductance oi' the device by a factor roughly proportional to the dielectrode constant. The output electrodes 6 REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,476,323 Rack July 19, 1949 2,502,479 Pearson Apr. 4, 1950 2,524,033 Bardeen Oct. 3. 1950 2,524,035 Bardeen et al Oct. 3, 1950 

